Some calculations... - 500 million accounts. - 1% conversion (they could 2% or higher) - 5 million conversions - $50/year Wooo hoo. $250 million. Ok, not a billion but not a bad start, all without burning developers or free users.
Charge businesses for business level accounts and analytics and you could get another $250M.
Twitter's problem isn't just a "Now" problem it's a management team that's following Facebook on their path to ad failure.